Maturation or processing of 28S ribosomal RNA was aberrant in the livers of male Fischer rats fed diets containing Aflatoxin B1 (1 ppm) for 2, 4 and 6 weeks. However, activity of nuclear DNA-dependent RNA polymerase I (which measures ribosomal RNA precursor synthesis) was not significantly decreased in rats fed this carcinogen-containing diet for 2 weeks. These results are in partial disagreement with acute injection studies carried out in this and other laboratories where 0.5 to 1.0 mg/kg of Aflatoxin B1 was injected into male Fischer rats and rats were sacrificed 16 hrs. later. Under these conditions, aberrant maturation or processing of 28S ribosomal RNA occurs, but marked depression of nuclear DNA-dependent RNA polymerase Form I was measured. Thus, under the much more physiological route of dietary feeding, defects occur in processing of ribosomal precursor RNA but not in transcription of this RNA. To investigate whether defective processing of ribosomal precursor RNA is related to the carcinogenic process, other structurally unrelated potent hepatocarcinogen will also be investigated. The effects of oral ingestion of 31-methyl-4-dimethylaminoazobenzene by dietary feeding as well as ingestion of diethylnitrosamine administered in drinking water on these same parameters of ribosmal RNA will be determined.